High pressure and high temperature apparatus

ABSTRACT

A design for high pressure/high temperature apparatus and reaction cell to achieve ˜30 GPa pressure in ˜1 cm volume and ˜100 GPa pressure in ˜1 mm volumes and 20-5000° C. temperatures in a static regime. The device includes profiled anvils ( 28 ) action on a reaction cell ( 14, 16 ) containing the material ( 26 ) to be processed. The reaction cell includes a heater ( 18 ) surrounded by insulating layers and screens. Surrounding the anvils are cylindrical inserts and supporting rings ( 30 - 48 ) whose hardness increases towards the reaction cell. These volumes may be increased considerably if applications require it, making use of presses that have larger loading force capability, larger frames and using larger anvils.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of the filing date of U.S.provisional patent application Serial No. 60/226,318 filed Aug. 21,2000.

STATEMENT OF GOVERNMENT SUPPORT OF THIS INVENTION

[0002] This invention was supported under the Department of Energy,Grant No, DE-FG02-96ER 82154

BACKGROUND AND SUMMARY OF THE INVENTION

[0003] This invention relates to apparatus for providing high pressureand temperature for use in the formation of minerals and new materials.

[0004] Conventional high pressure units enable pressures of ˜15 GPa(using WC/Co anvils) and ˜100 GPa (for diamond anvils) in a workingvolume of ˜1 μm³, but with a limitation in temperature of about 2,000°C. However, if a reaction cell could be made with a larger workingvolume and even higher temperature capabilities, there is then thepossibility of synthesizing diamonds directly from molten carbon in arelatively short time. Such synthesized diamond pieces will have finegrain size or single crystal structures, depending on the solidificationrate.

[0005] Previous apparatus for achieving high pressures and temperaturesmay be found in: P. W. Bridgman Scientific American, November. 1955,p.42;. U.S. Pat. No. 2,941,248 to H. T. Hall “High temperature-highpressure apparatus”; and. U.S. Pat. No. 3,746,484 to L. F. Vereshchaginet al “Apparatus for achieving high pressure and high temperature”.

[0006]FIG. 1 shows schematically the key components of the highpressure/high temperature apparatus of the present invention, and thecorresponding reaction cell (which holds the material to be processed),designed and implemented for the hot-pressing of carbon-based and othermaterials. The high pressure/high temperature apparatus consists of twoprofiled anvils 1 and three supporting steel rings 2-4 supporting eachanvil. The anvils 1 squeeze a container 5 made of plastic stone and areaction cell 6 that resides within the container. Cylindrical inserts 7and 8 are disposed above and below profiled anvils 1 and are constructedfrom WC/6 wt % Co, which are supported by steel rings which aredescribed in detail with below. The hardness of the supporting ringsdecreases from the center of the apparatus to the periphery. Reactioncell 6 consists of a graphite crucible that serves as a heater whenelectrical current is passed therethrough.

[0007] Supporting steel rings are used to increase the allowed loadexerted on the anvils and inserts. In effect, they provideside-supporting pressure, which increases the effective fracturestrength of the anvils under compression. A set of such supporting ringsis needed, since the maximal supporting pressure that a multilayercylinder can bear is twice the maximum pressure that can be achieved ina monolayer cylinder:

P _(o(max))≈2σ_(ts)/{square root}{square root over (3)}  (1)

[0008] where σ_(ts) is the ultimate tensile strength of the steel. It is˜2.0 GPa for hardened steel. This scheme permits a maximal workingpressure in the RC (P_(Wmax)) that is higher than the compressivefracture strength of the anvils; however, this pressure is always lessthan the Vickers hardness (H_(V)) of the anvils:

σ_(fs)≦P_(Wmax)≦H_(V)   (2)

[0009] The maximal working volume (V_(max)) that can be achieved underpressure depends on σ_(fs), maximal loading force (F_(max)) of press,size of frame window (a_(j)) and size of anvils used (V_(a)):

V _(max) =V _(max)(σ_(fs) , F _(max) , a _(j) , V _(a))   (3)

[0010] According to theory, the fracture compressive strength of abrittle material is inversely proportional to the sample volume:

σ_(fs)=ησ_(cs) V _(a) ^(−γ)  (4)

[0011] where η=η₀V_(0a) ^(γ) is constant, η₀ is dimensional constantthat is typical of a given material, V_(0a) is the volume of a standardsample for measuring compressive strength (σ_(cs)) and exponent γ is atypical value for a given material (γ˜{fraction (1/15)} for regularWC/Co). The values of σ_(cs) and H_(V) in formula (2) are alsointerrelated. The H_(V) ^(st) is ˜2.5σ_(cs) ^(st) for hardened steelthat has some plasticity. The H_(V) ^(cer) is ˜7σ_(cs) ^(cer) forbrittle rocks, stones and ceramics. The H_(V) ^(com) is about from 3 to5σ_(cs) ^(st) for composite materials with brittle skeleton and plasticmatrix, such as materials of the WC/Co type. The degree of sensitivityof the compressive fracture strength on sample volume depends onporosity, crystallite size, and value of side supporting pressure(P_(ss)):

γ=γ(ρ_(A) , d, P _(ss))   (5)

[0012] where ρ_(A) is apparent density, d is typical size ofcrystallites.

[0013] The high pressure/high temperature apparatus of the presentinvention enables a maximum possible static pressure over the range1-100 GPa Hereafter, we will call the range 1-10 GPa “very highpressure” and the range 10-100 GPa “super high pressure”. Even higherpressures in large volume can, in principle, be achieved with the helpof dynamic methods. We will call this pressure range (P>100 GPa) “ultrahigh pressure”.

[0014] Let us now consider how to achieve very high temperatures in thehigh pressure/high temperature apparatus. The necessary high temperatureis best realized by passing an electric current directly through thegraphite container. The thermal regime of the reaction cell and itscontainer may be computed from the following equation:

Wdt=∫cρ·dT·dV+(

λ·gradT·dS)dt   (6)

[0015] where W=qdV is power, q is power emitted in unit volume, c isspecific heat capacity, ρ is density, λ is thermal conductivity. Thisequation in the static state may be represented as:

div(λ·gradT)=0   (7)

[0016] An approximate solution of equation (6) for spherical thermalconductivity provides an opportunity to determine the thickness ofthermal insulation and the relaxation time that is needed for thereaction cell to respond to a power change and to achieve steady state:$\begin{matrix}{{{{T = {T_{\max}\left( {1 - e^{- \frac{i}{\tau}}} \right)}}{{{{where}\quad T_{\max}} = {W/v}};\quad {\tau = {\eta/v}};}v = {\int{{{div}\left( {\lambda \quad {grad}\quad {\psi (r)}} \right)}{V}}}};}{{\eta = {\int{{c \cdot {{\rho\psi}(r)}}{V}}}};}} & (8)\end{matrix}$

[0017] and ψ(r) is a function typical of a specific container. If theenergy released is not uniform over the entire volume of the sample,some interval of time will be necessary to heating the center of thesample to a temperature close to that of the heater, T_(max). A typicalrelaxation time (τ₀) depends on the materials properties and size ofsamples: $\begin{matrix}{\tau_{0} = {\frac{1}{3}\frac{c_{0}\rho_{0}}{\lambda_{0}}r_{0}^{2}}} & (9)\end{matrix}$

[0018] where c₀, ρ₀, λ₀ are heat capacity, density and thermalconductivity of sample and r₀ is radius of sample.

[0019] A temperature range of 100-2000° C. is achievable using graphiteheaters. In the temperature range 2000-4000° C., the carbon does notmelt, but reacts with all elements and compounds, with the exception ofinert gases. The very high temperature range, up to 4000-5000° C., isdifficult to obtain, especially under high pressure, because theefficiency of thermal insulation is limited.

[0020] Thermal flow from the heater is proportional to the first powerof temperature, but thermal flow by radiation is proportional to thefourth power of temperature:

dE _(e)=σ^(ST)

β _(T)(r)α_(T)(r)T ⁴ dSdt   (10)

[0021] where α_(T) is blackness, σ^(ST) is Stefan-Boltzman constant,β_(T) is integral coefficient of reflection of electromagnetic wavesfrom the surface. The energy dE_(e) added to the right side of eq. (6)increases the heat transfer at very high temperature and leads to asituation where a small increase in temperature demands a largeincrement in heater power.

[0022] The apparatus to be described below permits the achievement ofstatic super high pressure in combination with static very hightemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For a better understanding of the invention, reference is made tothe following drawings which are to be taken in conjunction with thedetailed description to follow in which:

[0024]FIG. 1 illustrates the key components of the high pressure andtemperature apparatus of the present invention;

[0025]FIG. 2 illustrates in detail the high pressure and temperatureapparatus of the present invention;

[0026]FIG. 3 is a sectional view of a reaction cell for use in the20-2000° C. temperature range;

[0027]FIG. 4 is a sectional view of a reaction cell for use in the20-5000° C. temperature range; and

[0028]FIG. 5 illustrates schematically the control arrangement of thehigh pressure and temperature apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029]FIG. 2 illustrates the high pressure and temperature apparatus 10of the present invention which is shown disposed between the upper 11and lower 12 steel contact plates of a conventional hydraulic press. Thecentral unit consists of upper and lower profiled anvils 14, 16 andsupporting annular steel rings 18, 20, 22 which surround the peripheryof each anvil 14, 16. Anvils 14, 16 press on a container 24 made ofplastic stone and a reaction cell 26 that resides within container 24.Upper and lower intermediate cylindrical inserts 28 are disposed aboveand below profiled anvils 14, 16 and outer cylindrical inserts 30 arelocated between inserts 28 and contact plates 11 and 12 of the hydraulicpress. The materials used to construct anvils 14, 16 and cylindricalinserts 28, 30 are chosen so that the hardness increases verticallytowards reaction cell 26. Contact plates 11, 12 are made of soft steel,outer inserts 30 are constructed of hardened steel and intermediateinserts 28 are made of a hard alloy. Anvils 14, 16 are manufacturedeither from diamond (HK=100 GPa) or from carbides (TiC or SiC) with ahardness of HV=30 GPa. Profiled anvils made from fine-structured TiC canachieve a pressure of 30 GPa in a volume of ˜1 cm³, and profiled anvilsmade from fine-structured Diamond can achieve a pressure of 100 GPa in avolume of ˜1 mm³. The inserts and anvils thus form a pyramidal structuresuch that the pressure decreases from a maximum inside reaction cell 26to a pressure of <1 GPa on the interface between the outer inserts 30and the plates 11, 12 of soft steel. The above mentioned volumes may beincreased by the use of a press with a larger frame, maximal loadingforce and larger anvils. The maximal loading force will rise asF_(max)˜P_(max)·V^(2/3).

[0030] Container 24 of plastic stone is located between two profiledanvils 14, 16. Some part of the container flows out of the cavity tofill the clearance between the anvils when the loading force isincreased, thus fixing the pressure gradient from maximum inside thereaction cell to ambient outside the container. The clearance betweenouter part of anvils 14, 16 and supporting rings 18, 20 and 22 may bevacuum, air (gas) or a polymeric material with high compressibility(such as rubber). Such a rubber ring placed around the container servesto regulate the pressure gradient. The hardness of the plastic stone ofcontainer 24 is 1 to 3 on the Mohs scale. It is generally the same forall volumes of containers or gradually decreases from the reaction cellto the periphery.

[0031] Anvils and inserts can be made from, for example, fine-structuredW—C—Co, W—Ti—C—Co—Fe—Ni, Ti—C, Si—C, Si—W—Ti—C—Co—Fe—Ni, and C. Inaddition to the fact that the hardness of the anvils and the cylindricalinserts increases towards the reaction cell, the anvils themselves canhave a functionally graded hardness, wherein the hardness graduallydecreases from the portion contacting reaction cell 26 towards inserts28. By way of example, profiled functionally graded anvils can be madefrom WC—TiC—Co alloy, where the quantity of TiC decreases from 100% onthe side contacting reaction cell 26 to 5% on the outer part; with thecorresponding Co content increasing up to 10% on the outer part of thecell. Such profiled anvils are capable of maintaining static pressure upto 100 GPa ( if the anvils are constructed of diamond) inside thereaction cell, with decrease to ambient pressure outside the reactioncell.

[0032] Steel annular support rings 32, 34. 36 surround the periphery ofintermediate inserts 28 and steel annular support rings 38, 40 and. 42surround the periphery of outer inserts 30. Support rings 18, 20, 22which surround anvils 14, 16; support rings 32, 34. 36 which surroundintermediate inserts 28 and support rings 38, 40, 42 which surroundouter inserts 30 are made from hardened alloyed steel. The heattreatment of the support rings is done in such a way that hardness andultimate tensile strength decrease from center to periphery, but theplasticity of the rings increases. The calculation of stresses in therings is done so that all the rings work in the elastic range and themaximal stress in any one ring does not exceed the yield strength(σ_(ys)) of the material. Outer safety rings 44, 46, 48 are made fromsoft, non heat-treated steel. Centering rings 50, 52, 54, 56, 58 and 60disposed outside safety rings 44, 46, 48 are made from non-conductingpolymeric materials and are used for precisely locating the axes of topand bottom parts of the apparatus.

[0033] High pressure and temperature apparatus 10 has a cylindrical axisof symmetry with the top and bottom parts are electrically insulatedfrom the press. As is seen, the hardness of the inserts and anvilsincreases in a vertical direction towards the reaction cell.Furthermore, the hardness of the supporting rings increases in theradial direction towards the reaction cell. The power supply isconnected to the top and bottom parts of the high pressure andtemperature apparatus 10 by copper cables. The electrical voltage can beapplied to the top and bottom parts of the high pressure and temperatureapparatus 10. The heater of the reaction cell is connected to the anvilsby contacting units.

[0034] The details of a reaction cell 26 for material processing in the20-2000° C. temperature range and its relationship to anvils 14, 16 isshown in FIG. 3. The profiled walls 70, 72 of anvils 14, 16 formcontainer 24 in which reaction cell 26 is located together with “plasticstone” which is thermally and electrically non-conductive material whichcan be, for example fine limestone (calcite), pyrophyllite, talc, clay,gypsum, or combination of that, or a mixture of clay and sand, or othernon conductive material. The edges of anvils 14, 16 can be sealed withan elastic, plastic or rubber ring 76. The profiled walls 70, 72 ofanvils 14, 16 form a central cavity 78 for receiving reaction cell 26which is constructed of graphite ceramic and which has a cylindricalaxis of symmetry in which the material to be processed 80 is placed. Thesurfaces of anvils are spherically shaped, so that the shape of cavity78 between the two anvils is close to spherical. This shape along withthe centering rings assures centering of the container material. Thegraphite ceramic forming reaction cell 26 becomes heated when anelectrical current is passed therethrough. The conductivity of thegraphite ceramic material is considerably higher (σ_(Ω)≅10⁺³ (ohm·cm)⁻¹)than that of the container material (σ_(Ω)10⁻¹² (ohm·cm)⁻¹). Reactioncell 26 is electrically connected to anvils by contacting units 82 whichare also formed from graphite ceramics or from metal foil.

[0035] The design of a reaction cell for processing in the 20-5000° C.temperature range is shown in FIG. 4. The material 86 to be processed islocated inside a cylindrical graphite ceramic heater 88 which issurrounded by a layer 90 of pure diamond powder, which insulates heater88 from a first cylindrical graphite ceramic screen 92 which isconcentric with heater 88. Screen 92 is insulated from a second graphiteceramic screen 94 by a layer of carbide powder 95, such as SiC or B₄C,which does not react with carbon over the temperature range existingbetween screens 92, 94 under steady state conditions. Screens 92, 94serve to reflect the radiant heat while the diamond 90 and carbide 95layers serve as electrical and chemical insulators preventing currentpassing trough the screens and chemical reactions between container 24and heater 88 up to 5000° C. Screen 94 is located inside container 24which again is made from plastic stone. A copper foil 96 is placed onthe anvil walls 70, 72 and provides the electrical contact to heater 88by means of graphite ceramic contacting units 97. A rubber ring 98around the outside of container 24 regulates the pressure gradient inthe container. This reaction cell permits melting material such ascarbon under super high pressure in a static regime and in a relativelylarge volume.

[0036] The overall design of a high pressure/high temperature apparatus99 is shown in FIG. 5; it consists of reaction cell 26 (as describedabove) inserted within container 24 of plastic stone disposed betweenanvils with supporting steel rings 100; inserts with supporting rings108 and insulating layers 109. Apparatus 99 includes a frame 102, ahydraulic ram 106 with its associated oil tank 116 joined by anelectrically operated valve 107. Hydraulic ram 106 is powered by an oilpump 104 operated by an electrical motor 103. Also acting on hydraulicram 106 is a smaller (up to 500 bar oil pressure capabilities) oil pump114 operated by an associated electrical motor 113. The uses of largeand small oil pumps permits precision control of the operating pressurewhich is difficult with only a single pump. The apparatus is controlledby a controller 105 (such as a programmed logic controller) which inturn is operated by a computer 115. Operation of the apparatus ismonitored by electrical multimeters 101 and 110 (such as Hewlett PackardHP 34401-A multimeters) and an oil pressure gauge 112. A 10 kilowattpower supply 117 operating through a high current 0.1 milliohm shunt 111supplies the power to heat reaction cell 26 and oil pump motors 103, 113and the other components are simply powered by 115 volt AC. Motors 103,113, valve 107, oil pressure gauge 112, multimeters 101, 110 and powersupply 117 are electrically joined with computer 115 through controller105. Computer 115 monitors oil pressure-voltage-current-time parametersand provides control signals to motors 103, 113; valve 107, and powersupply 117.

[0037] The present apparatus may also be used at pressures less than 1Gpa depending on the needs of the material to be processed. Furthermore,the operating pressure can be increased to above 100 Gpa and theoperating temperature increased above 5000° C. by dynamic methods.

[0038] The invention has been described with respect to preferredembodiments. However, as those skilled in the art will recognize,modifications and variations in the specific details which have beendescribed and illustrated may be resorted to without departing from thespirit and scope of the invention as defined in the appended claims

What is claimed is:
 1. High pressure/high temperature apparatus formaterial processing comprising: a) a pair of confronting profiledanvils, said confronting profiled anvils forming a cavity therebetween;b) a reaction cell for holding the material to be processed, saidreaction cell being disposed in the cavity formed between the profiledanvils; c) cylindrical inserts disposed above and below said anvils; andd) at least one annular supporting ring surrounding said profiledanvils.
 2. The high pressure/high temperature apparatus as claimed inclaim 1, wherein the reaction cell comprises a graphite crucible.
 3. Thehigh pressure/high temperature apparatus as claimed in claim 2, furtherincluding insulating material disposed about said graphite crucible. 4.The high pressure/high temperature apparatus as claimed in claim 1,further including second cylindrical inserts disposed above and belowsaid first cylindrical inserts.
 5. The high pressure/high temperatureapparatus as claimed in claim 4, wherein the anvils are constructed of amaterial that is harder than said first cylindrical inserts and saidfirst cylindrical inserts are constructed of a material that is harderthan said second cylindrical inserts.
 6. The high pressure/hightemperature apparatus as claimed in claim 1, wherein the anvils areconstructed of a material that has a graded hardness with the hardestportion of the anvil located proximate to the reaction cell and thesoftest portion of the anvil located proximate to the cylindricalinsert.
 7. The high pressure/high temperature apparatus as claimed inclaim 1, further including a second annular supporting ring surroundingsaid first annular supporting ring and a third annular supporting ringsurrounding said second annular supporting ring.
 8. The highpressure/high temperature apparatus as claimed in claim 7, wherein thefirst annular supporting ring is constructed of a material that issofter and more flexible than that of said anvils, the second annularsupporting ring is constructed of a material that is softer and moreflexible than that of said first annular supporting ring and the thirdannular supporting ring is constructed of a material that is softer andmore flexible than that of said second annular supporting ring.
 9. Thehigh pressure/high temperature apparatus as claimed in claim 1, whereinthe reaction cell comprises a cylindrical graphite ceramic container forholding the material to be processed, said container becoming heatedwhen an electrical current is passed therethrough; a first graphitescreen spaced apart from and surrounding said container; and a firstinsulating material disposed between said container and said firstscreen;
 10. The high pressure/high temperature apparatus as claimed inclaim 9, further including a second graphite screen spaced apart fromand surrounding said first graphite screen; second insulating materialdisposed between said first screen and said second screen.
 11. The highpressure/high temperature apparatus as claimed in claim 9 wherein thefirst insulating material comprises diamond powder.
 12. The highpressure/high temperature apparatus as claimed in claim 10 wherein thesecond insulating material comprises carbide powder.
 13. A reaction cellfor processing materials under high temperature and pressure comprising:a) a cylindrical graphite ceramic container for holding the material tobe processed, said container becoming heated when an electrical currentis passed therethrough; b) a first graphite screen spaced apart from andsurrounding said container; c) first insulating material disposedbetween said container and said first screen; d) a second graphitescreen spaced apart from and surrounding said first graphite screen; e)second insulating material disposed between said first screen and saidsecond screen.
 14. The reaction cell as claimed in claim 13 wherein thefirst insulating material comprises diamond powder.
 15. The reactioncell as claimed in claim 13 wherein the second insulating materialcomprises carbide powder.
 16. The reaction cell as claimed in claim 13wherein the second insulating material comprises silicon carbide powder.17. The high pressure/high temperature apparatus as claimed in claim 1,wherein the cavity formed by the confronting profiled anvils isgenerally spherical in configuration.
 18. The high pressure/hightemperature apparatus as claimed in claim 1, further including at leastone annular supporting ring surrounding said cylindrical inserts.